Method for making a low density multi-ply paperboard with high internal bond strength

ABSTRACT

Methods for improving the internal bond strength of paperboard with greater than 25 percent crosslinked fiber in at least one ply are described. In the methods, additives are added to the slurry in various combinations and order while maintaining the ionic demand of the slurry at less than zero. Paperboard with high ZDT, Scott Bond and Taber Stiffness is obtained.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Application No. 60/783,624filed Mar. 17, 2006.

FIELD

The present application relates to a method for increasing the bondstrength in a multi-ply paperboard that has high crosslinked cellulosefiber present in at least one of the plies.

SUMMARY

This application is directed to a method improving the internal bondstrength of paperboard with greater than 25 percent crosslinked fiber inat least one ply. In the method, additives are added to the slurry invarious combinations and order while maintaining the ionic demand of theslurry at less than zero. Paperboard with high ZDT, Scott Bond and TaberStiffness is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the pilot line.

DESCRIPTION

In single or multi-ply paperboard where the inner plies contain greaterthan approximately 25 percent crosslinked cellulose fiber the density ofthe stratum will drop below 0.4 g/cc. As a result, the internal bondstrength can drop so low as to not only be well below levels requiredfor converting the paperboard into packaging products but also below thelevel where conventional methods of increasing the internal strengthcannot provide enough increase to meet minimum levels needed forconverting. This effect can occur either in the entire structure or somefraction within the structure. The present application provides a methodfor increasing the internal bond of low density paperboard back into therange which is useable for converting.

In this application, the use of high concentrations of wet end additiveshave been demonstrated while producing low density paperboard.

A distinguishing characteristic of the present application is that atleast one ply of the paperboard, whether a single-ply or a multiple-plystructure, contains crosslinked cellulose fibers and strength enhancingadditives such as mechanically refined fiber, anionic and cationicstarches and other additives to offset the board strength lost by addingthe crosslinked cellulosic fibers. The crosslinked cellulosic fibersincrease the bulk density of the insulating paperboard characteristicsof the board. The paperboard also contains chemical pulp fibers. Asdefined herein, chemical pulp fibers useable in the present applicationare derived primarily from wood pulp. Suitable wood pulp fibers for usewith the application can be obtained from well-known chemical processessuch as the kraft and sulfite processes, with or without subsequentbleaching. Softwoods and hardwoods can be used. Details of the selectionof wood pulp fibers are well known to those skilled in the art. Forexample, suitable cellulosic fibers produced from southern pine that areuseable in the present application are available from a number ofcompanies including Weyerhaeuser Company under the designations C-Pine,Chinook, CF416, FR416, and NB416. A bleached Kraft Douglas Fir pulp (D.Fir), and Grande Prairie Softwood, all manufactured by Weyerhaeuser areexamples of northern softwoods that can be used. Mercerized fibers suchas HPZ and mercerized flash dried fibers such as HPZ III, bothmanufactured by Buckeye Technologies, Memphis Tenn., and Porosinier-J-HPavailable from Rayonier Performance Fibers Division, Jessup, Ga. arealso suitable for use in the present application when used withcrosslinked cellulose fibers. Other non crosslinked cellulose fibersinclude chemithermomechanical pulp fibers (CTMP), bleachedchemithermomechanical pulp fibers (BCTMP), thermomechanical pulp fibers(TMP), refiner groundwood pulp fibers, groundwood pulp fibers, TMP(thermomechanical pulp) made by Weyerhaeuser, Federal Way, Wash., andCTMP (chemi-thermomechanical pulp) obtained from NORPAC, Longview,Wash., sold as a CTMP NORPAC Newsprint Grade, jet dried cellulosicfibers and treated jet dried cellulosic fibers manufactured by theWeyerhaeuser Company by the method described in U.S. application Ser.No. 10/923,447 filed Aug. 20, 2004. These fibers are twisted kinked andcurled. Additional fibers include flash dried and treated flash driedfibers as described in U.S. Pat. No. 6,837,970,

Suitable crosslinking agents for making crosslinked fibers includecarboxylic acid crosslinking agents such as polycarboxylic acids.Polycarboxylic acid crosslinking agents (e.g., citric acid, propanetricarboxylic acid, and butane tetracarboxylic acid) and catalysts aredescribed in U.S. Pat. Nos. 3,526,048; 4,820,307; 4,936,865; 4,975,209;and 5,221,285 The use of C₂-C₉ polycarboxylic acids that contain atleast three carboxyl groups (e.g., citric acid and oxydisuccinic acid)as crosslinking agents is described in U.S. Pat. Nos. 5,137,537;5,183,707; 5,190,563; 5,562,740; and 5,873,979.

Polymeric polycarboxylic acids are also suitable crosslinking agents formaking crosslinked fibers. These include polymeric polycarboxylic acidcrosslinking agents are described in U.S. Pat. Nos. 4,391,878;4,420,368; 4,431,481; 5,049,235; 5,160,789; 5,442,899; 5,698,074;5,496,476; 5,496,477; 5,728,771; 5,705,475; and 5,981,739. Polyacrylicacid and related copolymers as crosslinking agents are described U.S.Pat. Nos. 5,549,791 and 5,998,511. Polymaleic acid crosslinking agentsare described in U.S. Pat. No. 5,998,511 and U.S. Pat. No. 6,582,553.CHB405, a citric acid crosslinked cellulose fiber and CHB505, apolyacrylic acid crosslinked cellulose, both commercially available fromWeyerhaeuser Company, Federal Way, Wash. were used in this work.

In single or multi-ply paperboard construction a mixture of wood pulpfibers and crosslinked cellulose fibers are used. In one embodiment thecrosslinked cellulosic fibers are present in at least one layer at alevel of 25 to 80 percent by total fiber weight of the ply. In anotherembodiment the crosslinked fibers are present at a level of 40 to 75percent by total fiber weight of the layer and in yet another embodimentthey are present at a level of 50 to 70 percent by total fiber weight ofthe layer.

Ionic Demand Balance

The technology relies on the ability to balance the ionic demand in thewet end of the paper machine such that 1) anionic polymeric materialscan be retained on the fibers and fines without excess remaining in thewater system, 2) the fibers and system do not pass through the zerocharge point which destabilizes retention and drainage 3) since pulpfibers are anionic, some cationic material can be added, however, addingtoo much cationic material without balancing the excess anionic demandwill either cause the fibers to flocculate reducing formation and/orcause the drainage to drop, impacting the runnability.

Each of the components used in the paperboard containing crosslinkedfiber in this disclosure has a specific charge density typicallymeasured by ionic demand titration. A Mutek PCD—Titrator was used forthe particle charge titration coupled with the PCD 02 Particle ChargeDetector for measuring the ionic demand of the component or fiberfurnish. The method was performed according to a procedure from A.E.Staley Manufacturing, a subsidiary of Tate and Lyle, Decatur, Ill. Themethod is as follows.

1. Turn the Mutek on using the power switch on the back of theinstrument.

2. Place 10 mL of a well mixed sample in the sample vessel. Insert theplunger and washer into the vessel. The sample consistency should be nomore than 0.83. Thick stock samples should be diluted.3. With the instrument turned on, the plunger should move up and downand a mV potential should be displayed. The sign of the potential (+ or−) indicates whether the sample is cationic (+) or anionic (−).4. Titrate the sample with the appropriate titrant until the mVpotential reads 0 mV (PolyDADMAC is the cationic polymer and is used totitrate anionic samples; PVSK or PESNa are the anionic polymers used totitrate the cationic samples). A buret or syringe can be used to deliverthe titrant to the sample. Titration should not be conducted with morethan 4 mL of titrant since higher volumes will give inaccuratemeasurements. If the sample requires more than 4 mL of titrant, thesample should be diluted or more concentrated titrant should be used.5. Record the amount of titrant used to titrate the sample. To calculatethe demand of the system, use the following equation:

“Ionic Demand” (ueq/L)=(mL titrant)×(% titrant dilution)×(sampledilution)

Ionic Demand refers to the amount of anionic or cationic charge requiredto neutralize the counter ion charge and is expressed in meq/g orueq/kg. For example, an additive with an ionic demand of +2.2 meq/g hasan anionic demand of 2.2 meq/g; an additive with an ionic demand of −1.8meq/g has a cationic demand of 1.8 meq/g. Specific components whoseionic demand was measured by the Mutek method are noted in Table 1,other component values are from suppliers.

TABLE I Component Ionic Demand Fully bleached Softwood kraft pulp~−0.015 meq/g total Fully bleached Softwood kraft pulp ~−0.0015 meq/gavailable CHB405 ~−0.43 meq/g total CHB405 ~−0.015 meq/g availableKymene ® 557H ~+2.2 meq/g Hercobond ® 2000 ~−1.8 meq/g STA-LOK ® 300~+0.3 RediBOND ® 3050 ~−0.19 meq/g RediBOND ® 2038 ~+0.24 meq/gSTA-LOK ® 330 ~+0.41 meq/g PPD M-5133 ~+16 meq/g GALACTASOL ® SP813D~+2.3 meq/g

With reference to the table, the difference between the total andavailable ionic demand represents the amount of charge that is internalto the fiber that is not accessible to polymers of molecular weightabove 300,000 g/mole. For papermaking, the available ionic demand ismore representative of the results obtained in practice than the totalionic demand.

The situation is further complicated in a paper machine wet-end wheredilution water from outside sources and/or wash water from pulp millbleaching stages contain ionic materials, (both dissolved anddispersed), is used to control consistency of the pulp slurry. Inintegrated mills where excess ionic materials are present, materialsadded to the pulp slurry to increase internal bond strength can beconsumed by the excess ionic materials. Also, the available ionic siteson pulp wilt also depend on how much refining has been done and on thebasic fiber morphology, i.e. the smaller the fiber or partial fiber thehigher available surface area, and therefore the higher available ionicdemand.

In general it may be stated that the fiber slurry is anionic to startwith and should remain anionic through the paper making process i.e. theionic demand of the slurry should less than zero.

Mechanically refined fiber can be added to the slurry to increase thestrength of the paperboard. In one embodiment the mechanically refinedfiber has a Canadian Standard Freeness of less than 125 mL CSF, a curlindex of ⅓ or less of the unrefined fiber and a kink angle of ½or lessof the unrefined fiber.

In one embodiment mechanically refined fiber is added to the slurryfollowed by the addition of an anionic starch and then followed byaddition of a cationic fixative. After each addition step the slurryionic demand is less than zero. The slurry is deposited on a foraminoussupport, dewatered forming a web and dried to form a paperboard.

In one embodiment the total starch level on dry fiber is from 50 to 120lb/t. In another embodiment the total starch level on dry fiber is from60 to 100 lb/t. In yet another embodiment the total starch level is 80to 90 lb/t.

Cationic fixatives such as cationic starch (e.g. STA-LOK® 300, STA-LOK®330 and RediBOND®2038) have a low anionic demand i.e. less than 1 meq/g.Other cationic additives such as Kymene®557H have a high anionic demand(+2.2 meq/g). In one embodiment the cationic fixative has an anionicdemand of greater than zero but less than one meq/g. In anotherembodiment the cationic fixative has an anionic demand of from 1 meq/gto 10 meq/g.

The paperboard of the present application may be one of severalstructures. In one embodiment the paperboard is a single ply structure,in another the paperboard is a two-ply structure and in yet anotherembodiment the paperboard is a multi-ply structure.

In the method, the addition order of the additive can vary. As statedearlier, in one embodiment, mechanically refined fiber is added to theslurry followed by the addition of an anionic starch and then followedby addition of a cationic fixative. After each addition step the slurryionic demand is less than zero. The slurry is deposited on a foraminoussupport, dewatered forming a web and dried to form a paperboard. Inanother embodiment mechanically refined fiber is added to the slurryfollowed by the addition of a cationic fixative and then followed byaddition of an anionic starch. After each addition step the slurry ionicdemand is less than zero. The slurry is deposited on a foraminoussupport, dewatered forming a web and dried to form a paperboard. In yetanother embodiment, mechanically refined fiber is added to the slurryfollowed by the addition of an anionic starch and then followed byaddition of first cationic fixative, followed by adding a secondcationic fixative. After each addition step the slurry ionic demand isless than zero. The slurry is deposited on a foraminous support,dewatered forming a web and dried to form a paperboard. In each case,the first cationic fixative may have an anionic demand of from 1 meq/gto 10 meq/g and the second fixant may have an anionic demand of greaterthan zero but less than 1.

Mechanically Refined Fiber and High Levels of Starch

Fiber and polymer binders were applied to low density board so thatinternal bond strength increases by 100% or more with 10% or lessincrease in density. The effect of refining on freeness and ionic demandis shown in Table 11.

TABLE II Effect Of Refining On Ionic Demand Kink Ionic CSF, Curl angle,Demand,* #Test Fiber Description mL Index °/mm meq/g 1 LV LodgepolePine - 720 0.25 92 −0.0008 Unrefined 2 LV Lodgepole Pine - EW 550 0.1046 −0.0069 3 LV Lodgepole Pine - EW 275 0.07 31 −0.0118 4 LV Doug. Fir -Unrefined 675 0.23 64 5 LV Doug. Fir - EW 85 0.07 28 −0.0114 6 LVLodgepole Pine - EW 65 0.05 18 −0.0167 7 LV Lodgepole Pine 33 0.05 21−0.0114 *Fiber only LV, Longview EW, Escher Wyss VB, Valley BeaterEach of the following Examples were generated as follows:

-   1. Handsheets formed using typical handsheet making equipment with    an extension to reduce the forming consistency.-   2. 250 gsm OD fiber.-   3. 60% CHB405 (crosslinked fiber), dispersed independently; several    methods were used interchangeably (Valley beater with no load, lab    disk refiner with 1-2 amps over no load and a pilot scale deflaker.    Mechanical dispersion was done to improve formation.-   4. 40% Douglas Fir refined to 400 ml CSF; pH was adjusted to 7.-   6. 4 #/t Aquapel sizing agent.-   7. 5 #/t Kymene®557H.-   8. 25 #/t cationic starch, (STA-LOK® 300)

The above formulation serves as a control; adjustments to the chemistryare noted in each Example.

As defined herein, mechanically refined fiber (MRF) is mechanicallyrefined wood pulp for example, Lodgepole Pine having a Canadian StandardFreeness <125 mL, a index ½ or less of unrefined starting fiber and akink angle of ½ or less of the unrefined starting fiber. Curl Index andkink angle were determined using a Fiber Quality Analyzer (FQA) aspublished in the Journal of Pulp and Paper Science 21(11):J367 (1995).Mechanically refined fiber can be generated to meet these criteria bydifferent refining methods which have different impact on conventionalfiber properties. Table III shows the effect on Z-direction tensile anddensity of various formulations with mechanically refined fiber. ZDT wasdetermined by TAPPI 541.

TABLE III Effect Of Mechanically Refined Fiber Addition On StrengthProperties Den- sity Change in ZDT, Δ ZDT, Description g/cc Density kPa% 100% D. Fir 0.628 138 498 1138 Control (as above) 0.264 40.22  10%valley beater mechanically 0.292 10.6% 95.38 137 refined fiber replacing10% D. Fir  5% Escher Wyss mechanically 0.274 3.7% 78.14 94 refinedfiber replacing 5% D. Fir  5% Escher Wyss mechanically 0.274 3.7% 110174 refined fiber replacing 5% D. Fir  5% Escher Wyss mechanically 0.259−1.9% 52.86 31.4 refined fiber replacing 5% D. Fir Control 0.232 24.8 5% Escher Wyss mechanically 0.238 2.5% 68.95 178 refined fiberreplacing D. Fir  5% mechanically refined fiber 0.242 4.3% 85.5 244replacing D. Fir (double disk refined) Control 0.255 40.22  5% ValleyBeater mechanically 0.265 3.9 69.81 73.5 refined fiber replacing D. Fir 5% Valley Beater mechanically 0.256 0.4 80.15 99.3 refined fiberreplacing D. Fir Δ indicates “change in”

Internal bond strength can be increased by replacing some of thecationic starch with a higher ionic strength molecule such as Kymene® asshown in the following example. 50% CHB405. 50% Lodgepole Pine refinedto 400 mL CSF.

10 #/t Kymene® 557H from Hercules.10 #/t Stalok 400 cationic starch from Staley.

Mechanically refined Lodgepole pine fiber refined at 50 ml CSF using anEscher Wyss laboratory refiner.

In this formulation the level of the Kymene®557H with an ionic demand of+2.2 meq/g was doubled and the STA-LOK® 300 cationic starch with anionic demand of +0.3 meq/g was reduced by 60%. As noted from the table,significant increases in ZDT bond strength and Scott Bond can beobtained by this method

TABLE IV Effect On Strength Of Partial Replacement Of Cationic StarchWith A Higher Ionic Demand Polymer Mechanically Scott refined fiberDensity % ZDT % Bond % % by wt. g/cc Increase kPa Increase J/m² Increase 0% 0.222 23 98 10% 0.234 +5.4% 62 +170% 135   +38% 20% 0.260 +14.6% 125+443% 173 +76.5%

A third technology is use of a starch excess. The general approach wasto overcome the normal limits of effective wet-end starch, balancing thecharge in the wet-end by adding excess anionic starch and fixing it tothe fibers by adding cationic starch or other high charge densitycationic polymers thus balancing the system to near neutral chargedensity. The neutralization was important to prevent excessiveflocculation and large impacts on drainage.

Specifically, total starch content added to the wet can be increased to2% to 5% based on dry fiber. Anionic starch such as RediBOND® 3050supplied by National Starch & Chemical or Aniofax® AP25 supplied byCarolina Starches can be used. Cationic fixatives include commoncationic starches like STA-LOK® 300 supplied by Staley Corp., PolyAluminum Chloride (PAC) like Nalco ULTRION® 8187 or high charge densitycationic polymers like M5133 and M5134, GALACTAOL® SP813D (anionic guar)and Kymene® 557H supplied by Hercules Corp. and Nalco NALKAT® 62060(branched EPEDMA) Nalco NALKAT® 2020 (poly DADMAC). As used herein, ahigh ionic demand is represented by a polymer that has an ionic demandof 1 meq/g to 17 meq/g, either as an anionic demand or as a cationicdemand. For example, Kymene®557H has an anionic demand of 2.2 meq/g andHercobond®2000 has a cationic demand of 1.8 meq/g.

The level of anionic starch needed to obtain high strength developmentdepends on the charge density and more importantly on the retention.Typically, 2% to 5% addition level based on dry fiber is adequate. Theamount of cationic fixative depends entirely on the size of the polymerand the cationic charge density. As defined herein, a fixative is acharged polymer that ionically bonds to a molecule of the oppositecharge. In general the higher the charge density the smaller the amountrequired and for equal charge density the larger the polymer the smallerthe amount required.

The following data was based on laboratory handsheets of the followingformulation:

-   1. Handsheets formed using typical handsheet making equipment with    an extension to reduce the forming consistency.-   2. 250 gsm OD fiber.-   3. 60% CHB405, dispersed independently; several methods were used    interchangeably, (Valley Beater with no load, lab disk refiner with    1-2 amps over no load condition, and a pilot scale deflaker).    Mechanical dispersion is done to improve formation.-   4. 40% Douglas Fir refined to 400 ml CSF.-   5. pH adjusted to 7.-   6. 5#/t Kymene® 557H.-   7. 4#/t Aquapel 625 sizing agent.-   8. 25#/t cationic starch (STA-LOK® 300)

Adjustments to the chemistry are noted in each Example. EXAMPLE 1

The above described handsheet is the control. The following adjustmentswere made to the non-fiber portion of the furnish, 80 lbs/t (4%)Aniofac® AP25 was mixed with the fibers, followed by 20 lbs/t cationicstarch STA-LOK® 300. Then Kymene®557H was added and the amount increasedto 10 lbs/t. Last, before sheet making a blend of cationic starchSTA-LOK® 300 and Aquapel 625 were added, the cationic starch was reducedto 20 lbs/t and the Aquapel was kept constant at 4 lbs/t. Handsheetswere evaluated for density, ZDT and Scott Bond the results are in TableV.

TABLE V Density ZDT Scott Bond Description g/cc kPa J/m² Control 0.24345 80 4% Anionic Starch Example 1 0.273 204 119 (Aniofac ® AP25)

EXAMPLE 2

The control handsheet as described above was adjusted as follows to thenon-fiber portion. 40 lbs/t Aniofax AP25 was mixed with the fibers,followed by 20 lbs/t STA-LOK® 300 cationic starch. Kymene® 557H at 5#/twas added and the same combination of Stalok 300 and Aquapel 625 as inExample 1, i.e. 20 lb/t and 4 lb/t, respectively. Handsheets wereevaluated for density, ZDT and Scott Bond; the results are in Table V1combined with the results from Example 1.

TABLE VI Density ZDT Scott Bond Description g/cc kPa J/m² Control 0.24345 80 4% Anionic Starch* Example 1 0.273 204 119 2% Anionic Starch*Example 2 0.262 85 113 *Aniofac ® AP25

EXAMPLE 3

The handsheet formulation described in Example 2 was altered to containmechanically refined fiber fibers so that the fiber portion of thefurnish is:

60% CHB405. 35% Fully Bleached D. Fir refined to 400 mL CSF. 5% Valleybeater mechanically refined fiber—fully bleached kraft Lodgepole Pine at˜50 mL CSF. The remainder of the additives are the same as in Example#2. The results are shown in Table V11.

TABLE VII Density ZDT Scott Bond Description g/cc kPa J/m² Control 0.24345 80 4% Anionic Starch* Example 1 0.273 204 119 2% Anionic Starch*Example 2 0.262 85 113 Control + 5% 0.274 78 106 Mechanically refinedfiber 2% anionic starch + 5% Example 3 0.283 122 148 mechanicallyrefined fiber *Aniofac ® AP25

EXAMPLE 4

Adjustments were made to the non-fiber portion of the of the handsheetformulation described in Example 3 (containing mechanically refinedfiber) as follows: 100 lb/t Aniofax® AP25 (was blended with the fibersfollowed by 90 lb/t Nalco 8187 PAC, then 5 lb/t Kymene® 557H, 5 lb/tSTA-LOK® 300 and 4 lb/t Aquapel 625. The results are shown in TableVIII.

TABLE VIII Density ZDT Scott Bond Description g/cc kPa J/m² Control0.243 45 80 Control + 5% mechanically 0.274 78 106 refined fiber 5%Anionic starch* + 4.5% Example 4 0.304 142 214 PAC *Aniofac ® AP25

EXAMPLE 5

Adjustments were made to the non-fiber portion of the of the handsheetformulation described in Example 3 (containing mechanically refinedfiber) as follows: 50 lb/t Aniofax® AP25 was blended with the fibersfollowed by 8 lb/t Nalco 62060 poly, then 5 lb/t Kymene® 557H, 5 lb/tSTA-LOK® 300 and 4 lb/t Aquapel 625. The results are shown in the TableIX.

TABLE IX Density ZDT Scott Bond Description g/cc kPa J/m² Control 0.24345 80 Control + 5% Mechanically 0.274 78 106 refined fiber   5% Anionicstarch* + 4.5% Example 4 0.304 142 214 PAC 2.5% Anionic starch* + 0.4%Example 5 0.284 109 138 Poly DADMAC *Aniofac ® AP25

EXAMPLE 6

Adjustments were made to the non-fiber portion of the of the handsheetformulation described in Example 3 (containing mechanically refinedfiber) as follows: 100 lb/t Aniofax® AP25 was blended with the fibersfollowed by 6 lb/t Nalco 2020 poly, then 5 lb/t Kymene® 557H, 5 lb/tSTA-LOK® 300 and 4 lb/t Aquapel 625. The results are shown in Table X.

TABLE X Density ZDT Scott Bond Description g/cc kPa J/m² Control 0.24345 80 Control + 5% Mechanically 0.274 78 106 refined fiber   5% Anionicstarch* + 4.5% Example 4 0.304 142 214 PAC 2.5% Anionic starch* + 0.4%Example 5 0.284 109 138 Poly DADMAC   5% Anionic Starch* + 0.3% Example6 0.279 218 215 Poly *Aniofac ® AP25

Single-ply handsheets designed to simulate the mid-ply of low densitymulti-ply paperboard were made. A 0.015 percent to 0.035 percentconsistency slurry was used in these studies. Handsheet making equipmentwas standard 8″×8″ sheet mold modified with an extended headbox so thattwice the normal volume of stock was used. This modification wasnecessary to improve handsheet formation when using materials designedto generate high bulk (e.g. crosslink fiber such as CHB405 and CHB505).Fiber weights are expressed as a weight percent of the total fiber dryweight; additives are based on weight of dry fiber.

A series of handsheets were made using different levels of wet-endadditives, different addition order and some changes in fiber furnish todemonstrate the level of internal bond strength that could be generatedby starch loading the web. The additives were added to the slurry in theorder across each sample row and the slurry stirred after each addition.

Series 1.

The Table XI below shows the conditions and formulations used whenmaking the series of handsheets

TABLE XI-A Handsheet Formulation And Addition Order. Anionic CationicAnionic Cationic PVOH Starch Starch Starch Starch Target CelaneseMechanically Avebe STA- Avebe STA- Basis D. Celvol Refined Aniofax LOK ®Aniofax Kymene ® LOK ® Aquapel wt. CHB405 Fir 165SF Fiber* AP25 300 AP25557H 300 650 Code g/m² % % % % #/t #/t #/t #/t #/t #/t 1 250 60% 30% 5%5% 0 0 0 5 25 4 2 250 60% 35% 0% 5% 0 0 0 5 25 4 3 250 60% 40% 0% 0% 4020 0 5 20 4 4 250 60% 35% 0% 5% 40 20 0 5 20 4 5 250 60% 35% 0% 5% 0 2040 5 20 4 6 250 60% 35% 0% 0% 80 20 0 10 10 4 7 250 60% 35% 0% 5% 80 200 10 10 4 8 250 60% 35% 0% 5% 0 10 80 15 10 4 9 250 60% 40% 0% 0% 0 0 05 25 4 *Lodgepole Pine refined with Valley Beater to 33 CSF

TABLE XI-B Calculated Ionic Demand As Chemical Additions Are Made InTable XI-A Ionic Ionic Ionic Ionic strength Ionic Ionic Ionic strengthdemand, strength with strength strength strength with Ionic Ionic IonicIonic total with STA- with with with STA- Aquapel demand demand demanddemand pulp and Aniofax ® LOK ® Aniofax ® Kymene ® LOK ® 650 - endCHB405 D. Fir PVOH MRF particles AP25 300 AP25 557H 300 point Code ueq/gueq/g ueq/g ueq/g ueq/g ueq/g ueq/g ueq/g ueq/g ueq/g ueq/g 1 −9 −0.45 0−0.57 −10.02 −10.02 −10.02 −10.02 −4.52 −0.89 −0.89 2 −9 −0.53 0 −0.57−10.1 −10.1 −10.1 −10.1 −4.60 −0.97 −0.97 3 −9 −0.6 0 0 −0.96 −14.2−11.3 −11.3 −5.8 −2.9 −2.9 4 −9 −0.53 0 −0.57 −10.1 −14.7 −11.8 −11.8−6.3 −3.40 −3.40 5 −9 −0.53 0 −0.57 −10.1 −10.1 −7.20 −11.8 −6.3 −3.40−3.40 6 −9 −0.53 0 0 −9.53 −18.7 −15.8 −15.8 −4.8 −3.38 −3.38 7 −9 −0.530 −0.57 −10.1 −19.3 −16.4 −16.4 −5.4 −3.94 −3.94 8 −9 −0.53 0 −0.57−10.1 −10.1 −0.86 −0.86 −1.34 0.11 0.11 9 −9 −0.6 0 0 −0.96 −0.96 −0.96−0.96 −4.1 −0.48 −0.48

Each handsheet was then coated with Polyvinyl Alcohol (PVA) coating,Celvol V24203 supplied by Celanese Ltd. The total coat weight was about50 g/m² and was divided equally between each side of the sheet. Thecoating was added to the surface to facilitate testing Z-directiontensile (ZDT) and internal Scott Bond because low density structureswithout the coating tend to separate at the tape instead of the withinthe sheet.

Each sheet was evaluated for several physical properties including basisweight, caliper, ZDT, internal Scott Bond and Taber Stiffness (15°).Scott Bond and Taber Stiffness were determined by TAPPI T 569 om-00 andT 489 om-04, respectively. Table XII below shows the results for thesekey characteristics

TABLE XII Physical Characteristics of Laboratory Handsheets as Describedin Table XI Basis Z-direction Scott Taber Weight Density Tensile BondStiffness Sample g/m² g/cm³ kPa J/m² g cm 1 297 0.267 124 158 375 2 2960.274 78 106 316 3 306 0.262 85 113 433 4 303 0.283 122 147 387 5 3030.281 121 142 388 6 305 0.273 204 119 403 7 301 0.284 179 128 374 8 3030.284 140 116 394 9 301 0.264 40 67 364

Samples 1, 2 and 9 can be considered the controls for this experiment.Sample number 9 is a fiber formulation designed to deliver low densitypaper and uses typical wet end chemistry (i.e. cationic starch andKymene®557H). The result is a very low Scott Bond, but typical TaberStiffness. Sample 2, incorporates mechanically refined fiber in aneffort to increase the internal bond and, by itself, results in anincrease in ZDT and Scott Bond, but not enough to reach the targetsneeded for converting multi-ply paperboard. It is estimated the minimumnecessary ZDT needed for converting is about 175-190 kPa.

Sample 1 incorporates mechanically refined fiber with a particlePVOH—known as a good binder but is hindered by issues with retention,cost and process reliability impacts. The increase in ZDT and Scott Bondfor sample 1 begins to approach the amount needed for convertingpaperboard. Samples 3 and 4 show that by adding 4% total starch to thefurnish the ZDT and Scott Bond essentially double. Adding mechanicallyrefined fiber, Sample 4, gives an increase of about the same magnitudeas it did to the original structure, Sample 2 v. Sample 4 and Sample 3v. Sample 9.

Sample 5 shows reversing the order (i.e. adding cationic starch firstthen anionic starch) in which the cationic and anionic starch are addedmakes no difference to the strength development

Samples 6 and 7 are a case where the amount of anionic starch is doubledwhile the cationic starch remains constant. Kymeme® 557H, a highercharge density cationic polymer, is used to balance the additionalanionic charge. The result is further increase in internal bond,increasing ZDT by 500%, (Sample 6) over the control and Scott Bond isunaffected by the additional starch and Kymene® 557H. Sample 7 showsthat by adding mechanically refined fiber the effect on ZDT is negativein this case, yet the Scott Bond increases.

Sample 8 adjusts the source of cationic charge further, increasing theamount of Kymene®557H and decreasing the amount of cationic starch. InTable XI-B the ionic demand of the system crosses from negative topositive at the last point of cationic starch addition and from TableXII the corresponding ZDT and Scott bond are further reduced indicatingthat when the ionic demand exceeds zero the effectiveness of the ionicbinding system is reduced.

The ZDT is reduced further, indicating that higher charge densitypolymer is less effective than cationic starch in adding internal bondstrength. In general the impact of the starch loading on Taber Stiffnessat 15° is small. For single ply handsheets this is reasonable becausecaliper is the dominating variable effecting bending stiffness. Theimpact of the starch loading on density is small enough that theincrease in elastic modulus of the sheets due to the starch loadingcompensates for the small changes in caliper. In a multi-ply web thesame response would be expected.

Series 2

A second set of handsheets was produced to determine the impact of usinghigh charge density cationic polymers to retain additional anionicstarch. It is thought that using higher charge density polymers lesstotal starch would be necessary to achieve the same strength due tobetter retention. The result would reduce the risk of affecting drainageand formation by adding excess starch. Table XIII shows the formulationsused in the experiments.

TABLE XIII Handsheet Formulation and Addition Order Anionic CationicCationic D. Fir @ Avebe STA- STA- CHB 500 ml Aniofax ® LOK ® Nalco NalcoNalco Kymene ® LOK ® 405 CSF MRF AP25 300 8187 62060 2020 557H 300 CodeWt. % Wt. % Wt. % #/t #/t #/t #/t #/t #/t #/t 1 60 35 5 80 20 5 5 2 6035 5 50 25 5 5 3 60 35 5 50 40 5 5 4 60 35 5 100 60 5 5 5 60 35 5 100 905 5 6 60 35 5 50 4 5 5 7 60 35 5 50 8 5 5 8 60 35 5 100 6 5 5 9 60 35 5100 12 5 5 10 60 35 5 50 4 5 5 11 60 35 5 50 8 5 5 12 60 35 5 100 6 5 513 60 35 5 100 12 5 5 14 0 100 0 5 25 All Codes at 250 g/m² target; alladditives are on a dry fiber wt. basis Aquapel 650 at 4 #/ton was usedin all the studies (dry fiber weight basis) MRF: Mechanically refinedFiber

Each handsheet was then coated with Polyvinyl Alcohol (PVA) coating,Celvol V24203 supplied by Celanese Ltd. The total coat weight was about50 g/m² and was divided equally between each side of the sheet. Thecoating was added to the surface to facilitate testing Z-directiontensile (ZDT) and internal Scott Bond, because low density structureswithout the coating tend to separate at the tape instead of the withinthe sheet.

Each sheet was evaluated for several physical properties including basisweight, caliper, ZDT, Internal Scott Bond, Taber Stiffness (15°) andother. The table below shows the results for these key characteristics

TABLE XIV Physical Characteristics Of Laboratory Handsheets As DescribedIn Table XIII Z-direction Taber Basis Weight Density Tensile Scott BondStiffness Code g/m² g/cm³ kPa J/m² g cm 1 306 0.287 186 132 392 2 3170.285 107 137 408 3 314 0.278 83 130 394 4 311 0.287 118 164 388 5 3200.304 142 214 400 6 303 0.286 87 122 373 7 309 0.284 109 138 392 8 3040.272 107 104 380 9 306 0.276 118 141 389 10 305 0.281 100 112 384 11308 0.282 103 141 394 12 308 0.279 218 215 390 13 307 0.276 122 109 39414 270 0.628 498 329 116 From Table XII - control with normal strengthadditives, as a reference. 9 301 0.264 40 67 364

Sample code 9 from Tables X1 and XII above is the base case.

The impact of using PAC such as Nalco 8187 (codes 2-5) to retain theanionic starch in the presence of mechanically refined fiber is lessthan that of using cationic starch on ZDT, however the impact on ScottBond is greater, suggesting that the PAC improves the retention of themechanically refined fiber giving greater shear strength to the board.

For codes 6-9 using NALKAT®62060 a branched EPEDMA cationic polymer as afixative, the impact at 2.5% anionic starch addition is roughly the sameas the PAC Nalco 8187) but significantly less ZDT development relativeto the cationic starch, Code 1. At the 5% anionic starch addition levelthere was no further significant gain

Use of the polyDADMAC (codes 10-13) as a fixative shows more promisethan the other two cationic polymers at the 5% added starch dose whereit exceeded (Code 12) the cationic starch in ZDT and Scott Bonddevelopment vs Code 1

The last code in Tables XIII and XIV, Code 14, is that of a normaldensity board, included for comparison. The higher ZDT and Scott Bondcome at the expense of bending stiffness.

In general, at equal total starch levels it appears that more ZDT isdeveloped when using combination of anionic and cationic starch thanwhen using higher charge density cationic polymers in combination withanionic starch. However, both methods develop significant ZDT. ScottBond has the opposite result. The shear strength appears to increase ata greater rate than the ZDT when using higher charge density cationicpolymers in combination with anionic starch.

Finally, the impact of the starch loading, independent of the cationicfixative has little effect on the product density and therefore littleimpact on the bending stiffness.

Pilot Trial

The disclosure was further explored using a pilot paper machine wheredynamic drainage and white water re-circulation could be used to improvethe simulation of commercial application on a paper machine.

The fiber furnish used in all of the following examples was the same,only the chemical additives and the order of addition were changed. Thefiber components were:

60% Weyerhaeuser CHB405

35% Weyerhaeuser fully bleached kraft D. Fir wet lap refined to ˜500 mLCSF

5% Douglas Fir refined to 85 ml CSF (Escher Wyss refined)

The chemical components were combinations of some or all of thefollowing, levels and addition order are shown in Table XV.

Kymene® 557H supplied by Hercules Incorporated (cationic wet strengthresin) Aquapel® 650 supplied by Hercules Incorporated (AKD sizing agent)Hercobond® 2000 supplied by Hercules Incorporated (anionicpolyacrylamide, retention aid) RediBOND® 3050 supplied by HerculesIncorporated (anionic starch) RediBOND® 2038 supplied by HerculesIncorporated (cationic starch) PPD M-5133 supplied by HerculesIncorporated (cationic high charge density polymer) GALACTASOL® SP813Dsupplied by Hercules Incorporated (cationic guar gum)

The pilot paper machine was a standard Fourdrinier type single plyformer. The design is such that there are several chemical additionpoints so that wet end additive effects can be studied. FIG. 1 shows thebasic unit operations with the chemical addition points. indicated aslower case letters as in Table XV. The addition points have been labeledand should be used as a reference for the formulations shown in TableXV.

Other unit operations were changed to maximize the bulk, for example,lowering the amount of vacuum on the forming table suction boxes,lifting the Dandy rolls away from the web using only one wet press, anormal drying profile, no size press (typical solids entering dryer32-36%) no calendaring and finally, samples were taken for evaluation atthe reel (eliminating effect of reel tension)

For each different formulation, the machine was run 10 to 15 minutesafter making adjustments and insuring the basis weight was on target. Inthis way, the white water was completely turned over and reachequilibrium with the new chemistry. Target basis weight was 200 g/m².

Each sample was then coated with Polyvinyl Alcohol (PVA) coating, CelvolV24203 supplied by Celanese Ltd. The total coat weight was about 22 g/m²and was divided equally between each side of the sheet. The coating wasadded to the surface to facilitate testing Z-direction tensile (ZDT) andInternal Scott Bond, because low density structures without the coatingtend to separate at the tape instead of the within the sheet.

TABLE XV Wet End Additives for Pilot Paper Machine Starch LoadingTrials. Code b c d e g h 1 5 #/t Kymene ® 2 #/t Hercobond 10 #/tRediBOND 2038 4.5 #/t Aquapel 650 557H 2000 2 20 #/t 20 #/t RediBOND 5#/t 2 #/t Hercobond 10 #/t RediBOND 2038 4.5 #/t Aquapel 650RediBond2038 3050 Kymene ® 2000 557H 3 20 #/t RediBond 20 #/t RediBOND 5#/t 10 #/t RediBOND 2038 4.5 #/t Aquapel 650 2038 3050 Kymene ® 557H 430 #/t RediBond 40 #/t RediBOND 5 #/t 2 #/t Hercobond 10 #/t RediBOND2038 4.5 #/t Aquapel 650 2038 3050 Kymene ® 2000 557H 5 20 #/t RediBond60 #/t RediBOND 10 #/t 2 #/t Hercobond 10 #/t RediBOND 2038 4.5 #/tAquapel 650 2038 3050 Kymene ® 2000 557H 6 20 #/t RediBond 10 #/tKymene ® 20 #/t 2 #/t Hercobond 10 #/t RediBOND 2038 4.5 #/t Aquapel 6502038 557H RediBOND 2000 3050 7 30 #/t RediBond 10 #/t Kymene ® 40 #/t 2#/t Hercobond 10 #/t RediBOND 2038 4.5 #/t Aquapel 650 2038 557HRediBOND 2000 3050 8 40 #/t RediBond 20 #/t RediBOND 5 #/t 10 #/tRediBOND 2038 4.5 #/t Aquapel 650 2038 2038 Kymene ® 557H 9 40 #/t 2 #/t8.2 #/t 10 #/t RediBOND 2038 4.5 #/t Aquapel 650 RediBOND 3050 M-5133Hercobond 2000 10 5 #/t Kymene ® 2 #/t Hercobond 10 #/t RediBOND 20384.5 #/t Aquapel 650 557H 2000 11 30 #/t 10 #/t Kymene ® 40 #/t 2 #/tHercobond 10 #/t RediBOND 2038 4.5 #/t Aquapel 650 RediBOND 2038 557HRediBOND 2000 3050 12 30 #/t 5 #/t 40 #/t 2 #/t Hercobond 10 #/tRediBOND 2038 4.5 #/t Aquapel 650 RediBOND 2038 Kymene ® 557H RediBOND2000 3050 13 30 #/t 2.5 #/t Kymene ® 40 #/t 2 #/t Hercobond 10 #/tRediBOND 2038 4.5 #/t Aquapel 650 RediBOND 2038 557H RediBOND 2000 305014 30 #/t 40 #/t RediBOND 5 #/t 2 #/t Hercobond 10 #/t RediBOND 2038 4.5#/t Aquapel 650 RediBOND 2038 3050 Kymene ® 2000 557H 15 8 #/t 40 #/tRediBOND 5 #/t 2 #/t Hercobond 8 #/t GALACTASOL 4.5 #/t Aquapel 650GALACTASOL 3050 Kymene ® 2000 SP813D SP813D 557H 16 40 #/t 20 #/tRediBOND 5 #/t 4 #/t GALACTASOL 4.5 #/t Aquapel 650 RediBOND 3050 2038Kymene ® SP813D 557H Lower case letters refer to the additive additionpoints in FIG. 1Codes number 1 and 10 are controls for two different running days, code14 is a repeat of code 4 on a different day and code 11 is a repeat ofcode 7 on a different day. The physical characteristics of the resultantpaper are shown in Table XV.

TABLE XVI Physical Characteristics of Pilot Paper Machine Samples inTable XV Geometric Geometric Basis Z-direction Mean Scott Mean TaberWeight Density Tensile Bond Stiffness Code g/m² g/cm³ kPa J/m² g cm 1228 0.271 161 198 135 2 256 0.273 194 242 163 3 251 0.283 204 258 161 4245 0.274 219 252 166 5 237 0.284 265 236 147 6 236 0.276 241 227 146 7239 0.279 232 236 146 8 262 0.288 199 246 190 9 236 0.260 134 189 156 10231 0.271 192 227 140 11 230 0.295 350 354 127 12 229 0.290 360 340 13113 234 0.288 319 323 131 14 244 0.292 339 340 144 15 228 0.288 305 297134 16 243 0.273 269 345 146

Pilot Machine Versus Control

The effect of starch loading is basically the same, it is estimated thatthe target internal bond strength that would be enough for performanceduring converting is about 2× of the control samples. For the pilottrial ZDT doubled and Scott Bond increased 75%.

By loading the wet-end with between 2% and 4% total starch (anionic andcationic), ZDT can be increased by approximately 25% to 85% relative tothe same furnish with conventional levels of cationic starch (Codes 2-8,11-14).

Loading up to 2% anionic starch into the wet-end and using high chargedensity cationic polymer (code 9) to retain the starch little or no gainin ZDT or Scott Bond was achieved.

Loading the wet-end with up to 2% anionic starch and using cationic guargum (codes 15 and 16) to improve retention as a substitute for cationicstarch about 40%-50% increase in ZDT was obtained.

When changing the order of addition, indications were that adding theanionic starch after the cationic material resulted in better strengthefficiency.

Starch loading resulted in an increase in density of <10% in all casesand had no significant impact on stiffness.

1. A method for forming at least one ply of a paperboard comprising thesteps of: forming a slurry of cellulose fibers comprising crosslinkedfibers; adding mechanically refined fiber; adding an anionic starchsubsequent to adding said mechanically refined fiber; adding a cationicfixative subsequent to adding said anionic starch; wherein, after eachaddition step, the slurry ionic demand is less than zero; depositingsaid slurry on a foraminous support; forming a fibrous web layer bywithdrawing liquid from said slurry; drying said web to form apaperboard.
 2. The method of claim 1 wherein said crosslinked fibers arepresent at a level from 25 to 80 percent of the total fiber weight in atleast one ply of said paperboard.
 3. The method of claim 1 wherein thetotal starch level is 50 to 120 lb/t.
 4. The method of claim 1 whereinthe mechanically refined fiber has a CSF of less than 125 CSF, a curlindex of ⅓ or less of the unrefined fiber and a kink angle of ½ or lessof the unrefined fiber.
 5. The method of claim 1 wherein the cationicfixative has an anionic demand of greater than zero but less than 1meq/g.
 6. The method of claim 1 wherein the cationic fixative has ananionic demand of from 1 meq/g to 10 meq/g.
 7. The method of claim 1,wherein said paperboard is at least a two-ply board, said at least oneply containing said crosslinked fibers.
 8. The method of claim 1,wherein said paperboard is at least a three-ply board, said at least oneply containing said crosslinked fibers.
 9. A method for forming apaperboard comprising the steps of: forming a slurry of cellulose fiberscomprising crosslinked fibers; adding mechanically refined fiber, addinga cationic fixative and mixing with said slurry; adding an anionicstarch subsequent to adding said cationic fixative; wherein, after eachaddition step, the slurry ionic demand is less than zero; depositingsaid slurry on a foraminous support; forming a fibrous web layer bywithdrawing liquid from said slurry; drying said web to form apaperboard.
 10. The method of claim 9 wherein said crosslinked fibersare present at a level from 25 to 80 percent of the total fiber weightin at least one ply of said paperboard.
 11. The method of claim 9wherein the total starch level is 50 to 120 lb/t.
 12. The method ofclaim 9 wherein the mechanically refined fiber has a CSF of less than125 CSF, a index of ⅓ or less of the unrefined fiber and a kink angle of½ or less of the unrefined fiber.
 13. The method of claim 9 wherein thecationic fixative has an anionic demand of greater than zero but lessthan 1 meq/g.
 14. The method of claim 9 wherein the cationic fixativehas an anionic demand of from 1 meq/g to about 10 meq/g.
 15. The methodof claim 9, wherein said paperboard is at least a two-ply board, said atleast one ply containing said crosslinked fibers.
 16. The method ofclaim 9, wherein said paperboard is at least a three-ply board, said atleast one ply containing said crosslinked fibers.
 17. A method forforming at least one ply of a paperboard comprising the steps of:forming a slurry of cellulose fibers comprising crosslinked fibers;adding mechanically refined fiber; adding an anionic starch subsequentto adding said mechanically refined fiber; adding a first cationicfixative subsequent to adding said anionic starch; adding a secondcationic fixative subsequent to adding said first cationic fixative;wherein, after each addition step, the slurry ionic demand is less thanzero; depositing said slurry on a foraminous support; forming a fibrousweb layer by withdrawing liquid from said slurry; drying said web toform a paperboard.
 18. The method of claim 17 wherein said crosslinkedfibers are present at a level from 25 to 80 percent of the total fiberweight in at least one ply of said paperboard.
 19. The method of claim17 wherein the total starch level is 50 to 120 lb/t.
 20. The method ofclaim 17 wherein the mechanically refined fiber has a CSF of less than125 CSF, a curl index of ⅓ or less of the unrefined fiber and a kinkangle of ½ or less of the unrefined fiber.
 21. The method of claim 17wherein the first cationic fixative has an anionic demand of from 1meq/g to 10 meq/g.
 22. The method of claim 17 wherein the secondcationic fixative has an anionic demand of greater than zero but lessthan 1 meq/g.
 23. The method of claim 17, wherein said paperboard is atleast a two-ply board, said at least one ply containing said crosslinkedfibers.
 24. The method of claim 17, wherein said paperboard is at leasta three-ply board, said at least one ply containing said crosslinkedfibers.